Processes for preparing a flexible polyurethane foam by reacting a polyisocyanate, one or more polyether polyols, and water have been described widely. One of the disadvantages of the prior art processes is that the blowing efficiency is not optimal. This means that either part of the water used does not react with the polyisocyanate and hence no CO2 is liberated or the CO2 is liberated too early and leaves the reacting mixture without effectively contributing to the foam expansion. Therefore, the density is often not as low as it could be. Another disadvantage is that at high stoichiometric water levels the foam properties such as hysteresis and related compression set properties deteriorate. Further, flexible polyurethane foams so prepared often do not have sufficient load-bearing properties. In order to provide such foams with enhanced load-bearing properties, often polyols are used that contain particulate materials dispersed therein. Examples of such polyols are so-called SAN-based polymer polyols, PIPA-polyols, and PHD-polyols. If the particulate material has particles with a rather big average particle size, often foam collapse is observed.
The formation of relatively small (up to 0.3 μm) urea aggregates in flexible polyurethane foam preparation in itself is known; see Journal of Applied Polymer Science, Vol. 35, 601-629 (1988) by J. P. Armistead et al. and Journal of Cellular Plastics, Vol. 30, page 144, (March 1994) by R. D. Priester et al. Until recently it was believed that by increasing the urea hard phase content other important properties like resiliency, hysteresis and compression set will suffer; see Polyurethanes Expo '98, 17-20 Sep. 1998, page 227 by D. R. Gier et al.